This invention relates to an improved process for the preparation of aromatic acids from alkyl-substituted aromatic hydrocarbons. More particularly, the invention relates to an improved process for preparing aromatic acids, such as naphthalene-2,6-dicarboxylic acid or terephthalic acid, by means of a NO.sub.x -mediated oxidation of alkyl side chains on an aromatic nucleus. In the case of terephthalic acid, for example, the invention relates to the NO.sub.x -mediated oxidation of the methyl groups of p-xylene to carboxyl groups.
Organic acids, such as naphthalene-2,6-dicarboxylic acid or terephthalic acid, are highly useful in the manufacture of polyester-type and polyamide-type resins, including polyethyleneterephthalate. Such organic acids customarily have been prepared by a liquid phase oxidation of an alkyl-substituted aromatic precursor in the presence of an aqueous acidic reaction medium, and usually in the presence of a metal catalyst. For example, U.S. Pat. No. 2,766,281 discloses the preparation of phthalic acids, including terephthalic acid, by a step-wise oxidation of a corresponding xylene with concentrated nitric acid at atmospheric pressure and in the presence of an inert organic reaction medium that is liquid at the reaction temperatures. In the first step, the xylene starting material is reacted with the nitric acid at a temperature of from about 120 .degree. C. up to the boiling point of the xylene starting material, while removing water by distillation, to form a toluic acid. In the second step, nitric acid is slowly added to the toluic acid at a temperature in excess of about 140 .degree. C., while removing water by distillation, to form the product phthalic acid.
Another patent that discloses the use of nitric acid as the oxidizing agent for converting p-xylene to terephthalic acid is U.S. Pat. No. 3,235,587. In that patent, it is disclosed to add excess nitric acid having a concentration of between 5% and 15% into a reactor, under superatmospheric pressure and at temperatures between 1500 .degree. C. and 200 .degree. C., and then to add the p-xylene into the reactor. The use of catalytic amounts of copper powder and ammonium metavanadate is said to facilitate the oxidation process.
Using oxygen or an oxygen-containing gas as the oxidizing agent in place of nitric acid has been disclosed. For example, U.S. Pat. No. 3,626,001 discloses a two-stage process for preparing terephthalic acid or isophthalic acid from the corresponding xylene. In the first stage, a feed comprising the xylene, an alkanoic acid solvent, a molecular oxygen-containing gas and a cobalt catalyst are heated to form a first stage product comprising substantial amounts of toluic acid. In the second stage, the first stage product is heated in a mixture with a alkanal corresponding to the alkanoic acid of the first-stage feed, a molecular oxygen-containing gas and a cobalt catalyst. Preferably, the alkanoic acid used in the first stage of the process is acetic acid, and the alkanal used in the second stage is acetaldehyde.
Numerous others patents discloses metal catalyzed processes which use acetic acid as the reaction medium during the oxidation of alkyl-substituted aromatic compounds to their corresponding acids. See, for example, U.S. Pat. Nos. 3,686,293; 4,145,560; 4,605,763; and 5,557,009, the disclosures of which are incorporated herein by reference.
U.S. Pat. No. 3,532,746 relates to a multi-stage process for preparing aromatic polycarboxylic acids having at least four carboxylic acid groups as nuclear substituents in at least two vicinal pairs. The process involves oxidizing an acid-precursor with molecular oxygen in the presence of an organic solvent, preferably acetic acid, bromine and a heavy metal oxidation catalyst, and in the further presence of an oxidation catalyst providing NO having an oxidation potential of -1.0 to -0.5 selected from nitric acid and inorganic nitrates and nitrites, such as calcium nitrate, sodium nitrite, barium nitrite and ammonium nitrate.
U.S. Pat. No. 3,219,691 relates to the preparation of pure naphthalene-2,6-dicarboxylic acid from an impure mixture of naphthalene dicarboxylic acids prepared by the liquid phase partial oxidation of mixed naphthalenes at a temperature above 140 .degree. C. by means of NO.sub.2 in the presence of a selenium catalyst. The process of preparing the impure acid mixture involves adding selenium to an alkyl naphthalene starting material in a trichlorobenzene solvent and then bubbling NO.sub.2 into reaction mixture at a temperature of 225 .degree. C. until a 50% excess of NO.sub.2 has been added. The reaction mixture is then washed successively with trichlorobenzene, pentane and water at room temperature to form an impure acid mixture containing on the order of 85% diacids by weight.
The use of N.sub.2 O as the stoichiometric oxidant at high concentrations (up to 20% of the gas mixture) for the conversion of benzene to phenol is known. See, for example, Appl. Catal. A 1992, 82,31.
While prior art processes have been successful in preparing aromatic polycarboxylic acids, in general, and phthalic acids, e.g., terephthalic acid, in particular, they generally have required the use of expensive catalysts and/or the use of significant quantities of acetic acid or other solvent, some of which is consumed and needs to be replaced.